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Phase-velocity magnetic resonance (PV-MR) quantifies differential pulmonary blood flow as accurately as the previous gold standard lung perfusion scintigraphy in patients with a single pulmonary blood source supplied by a subpulmonary ventricle (1–3). Therefore, in routine clinical practice, PV-MR has taken on the role of quantifying differential pulmonary blood flow in congenital heart disease (4). However, the interstudy variability and a benchmark of a clinically relevant change of repeat routine differential pulmonary blood flow measurements in congenital heart disease using PV-MR are still unknown.

To test the interstudy variability, we studied 80 consecutive routine clinical cases without intervention or pathological event of the pulmonary branch arteries between 2 consecutive PV-MR measurements (control group). Therefore, in this group, no change of the measured differential pulmonary blood flow ratio was to be expected. This group consisted of 56 cases with repaired tetralogy of Fallot, pulmonary atresia, or truncus arteriosus communis, 7 cases with atrial switch operation, 6 cases with arterial switch operation, 5 cases with Ross operation, and 6 cases with other congenital heart defects. Mean age was 23.2 ± 9.8 years at the time of the first measurement. Mean time between the 2 measurements was 1.2 ± 0.6 years.

To establish a benchmark of a clinically relevant change of repeat routine differential pulmonary blood flow measurements using PV-MR, we studied 13 consecutive routine clinical cases with an explicit unilateral intervention or morphological change to 1 of the pulmonary branch arteries between 2 PV-MR measurements (intervention group). Therefore, in this group, a clear change of the measured differential pulmonary blood flow ratio was to be expected. It is important to note that in our center, PV-MR measurements are not used to decide whether a pulmonary branch artery needs treatment. This group consisted of 11 cases with stent implantation or balloon dilation of 1 of the pulmonary arteries and 2 cases of a post-operative hematoma compressing the left pulmonary artery. Mean age was 21.9 ± 8.3 years at the time of the first measurement. Mean time between the 2 measurements was 0.8 ± 1.0 years.

In each of the measurements, we evaluated net flow volumes in the right and left pulmonary arteries. The representative entity for the differential pulmonary blood flow ratio was defined as percent right pulmonary blood flow as previously described (1) and calculated using the following equation: percent right pulmonary blood flow = 100 × [RPA net flow volume/(RPA net flow volume + LPA net flow volume)], where RPA and LPA are the right and left pulmonary arteries, respectively.

Bland-Altman analysis determined the 95% limits of agreement between 2 consecutive PV-MR measurements (5). The difference between 2 consecutive PV-MR measurements was calculated by subtracting percent right pulmonary blood flow at the first time point from the second time point.

In the control group, the mean difference between the first and second measurements of percent right pulmonary blood flow was −2% with 95% limits of agreement of 8% and −11% (Fig. 1). In contrast to this, in the intervention group, the differences between the 2 measurements of percent right pulmonary blood flow were all located outside of the 95% limits of agreement of the control group (Fig. 2).

Mean difference between 2 consecutive percent right pulmonary blood flow measurements: −4%; 95% limits of agreement: 42% and −50%. Note: all measurements of the intervention group lie outside of the 95% limits of agreement of the control group (grey zone).

Furthermore, in the intervention group, the absolute change of percent right pulmonary blood flow was significant at 22 ± 8% (p < 0.001), consistent with the side on which the procedure was performed and the absolute change of the pressure gradient of 22 ± 17 mm Hg (p = 0.029).

The results of this study show that consecutive routine clinical differential pulmonary blood flow measurements in congenital heart disease cases without intervention of the pulmonary branch arteries are highly reproducible by PV-MR, with an interstudy variability of up to about 10% (control group, Fig. 1). Moreover, we can use the results of our study to establish a benchmark of a clinically relevant change of repeat routine differential pulmonary blood flow measurements in congenital heart disease using PV-MR in a “real-life” setting (intervention group, Fig. 2). The 95% limits of agreement of repeat routine clinical measurements of percent right pulmonary blood flow by PV-MR of cases without intervention or pathological event of the pulmonary branch arteries (control group) were 8% and −11%. In contrast to this, all measured values in the intervention group were outside of these 95% limits of agreement of the control group. Consequently, we suggest that by using the deviations of 8% and −11% as benchmarks for measurement reproducibility, patients with a clinically relevant change (as assessed by the interventionist) of their differential pulmonary blood flow ratio will correctly be recognized in 100% of the cases. Moreover, patients without change of their differential pulmonary blood flow ratio will correctly be recognized in 95% of the cases.